1. Field of the Invention
This invention relates to the cleaning of tubes in heat exchangers. More particularly, a two-member cleaning device having an effective range of properties is provided to be pumped through the tubes while the heat exchanger is on-line.
2. Description of Related Art
Electrical power generation plants use steam-driven turbines to drive the electrical generators. A heat source, either a fossil fuel or a nuclear reactor, is used to produce the steam. The steam, after it has passed through the lowest-pressure turbine, is condensed in the shell of a tube-and-shell heat exchanger by cooling water passing through the tubes. The efficiency of heat removal from the steam by the cooling water determines the back-pressure at the low-pressure turbine exhaust, and this pressure significantly affects the total energy extracted from the steam. Since fuel costs represent approximately 95% of the total production cost of electricity, energy lost by not extracting energy from the steam leads directly to an increase in power cost.
Steam condensers in power plants contain thousands of thin-walled tubes for heat transfer. For example, in one 750 megawatt lignite-fired unit, the steam condenser contains 28,512 1-inch stainless steel tubes. Water flows through the heat exchanger at a rate of about 488,000 gallons per minute. In some steam condensers the tubes are made of a copper alloy.
To maintain maximum heat transfer efficiency, it is necessary to minimize build-up of a film, which decreases heat transfer, on the internal surface of the tubes of the heat exchanger. Techniques have been developed to remove films, at least to some degree, by periodically passing "tube cleaners" through the tubes while a heat exchanger is on-line. Ideally, the tube cleaners will circulate through all tubes at about the same frequency. In other words, the distribution of cleaners over the many tubes should be uniform.
While the discussion of heat exchangers, cooling water and cleaning of heat exchangers will be centered on steam condensers in power plants, it should be understood that the same problems with maintaining clean tubes in heat exchangers occur in other industries, such as the petroleum refining, petrochemical and chemical industries. The article "On-Line Mechanical Cleaning of Heat Exchangers," published in Hydrocarbon Processing, January 1983, describes mechanical cleaning of heat exchangers used in the refining industry.
The cooling water in heat exchangers may be contained in a closed system, where the water is air-cooled and recirculate back through the heat exchanger, or it may be drawn from fresh-water lakes or the sea. In some power plants the water is drawn from a fresh-water lake which is connected to the power plant through canals. The rate of fouling of condenser tubes by the water will vary greatly depending on whether conditions are conducive to growth of organisms and whether the chemical composition of the cooling water is such that chemical scales or deposits can form under conditions in the tubes. Biological fouling in steam condensers is commonly caused by bacteria and algae. Common chemical scales causing fouling include calcium carbonate, calcium sulfate, silica and manganese. Electrical utilities have found that chemicals such as chlorine can be used to decrease biofouling and scale inhibitors can be used for chemical scales, but environmental concerns limit the use of chemicals and make it especially important to find improved mechanical cleaning systems which can be used to at least supplement and, preferably, to supplant chemical methods.
There are two types of cleaning systems for the tubes of heat exchangers. Many patents and publications describe various mechanical devices and systems for off-line cleaning, i.e., cleaning the tubes when the heat exchanger has been placed out of service. Fouling occurs at such a rate in many heat exchangers, however, that an on-line system for cleaning is desirable. It is not possible in these instances to maintain high efficiency heat transfer without some form of control of fouling between shut-downs of the unit.
U.S. Pat. No. 5,083,606 discloses an endoscopic method of examination of condenser tubes while on-line and provides a good review of the components of a steam-electric plant, steam condensers and fouling of condenser tubes.
The installation of automatic on-line mechanical cleaning in power plants as early as 1966 in the U.S. has been reported in Chapter 2 of "Condenser Biofouling Control Symposium Proceedings," Ann Arbor Science Publishers, Inc., Ann Arbor, Mich., 1980. The performance of prior mechanical systems for condenser cleaning has been reviewed in a report of the Electric Power Research Institute, "Performance of Mechanical Systems for Condenser Cleaning," EPRI CS-5032, January 1987. The two common types of on-line cleaning bodies are sponge rubber balls (used in the Taprogge system, in which the balls are continuously circulated through tubes of heat exchangers, which is used in about one-third of North American installations) and brushes. In any type of on-line system, it is necessary to have a mechanism for removing the cleaning bodies and re-circulating them back through the heat exchanger tubes.
Sponge rubber tube cleaners are used by injecting the cleaners into the cooling water upstream of a heat exchanger and removing them downstream of the heat exchanger by a strainer. A pump is used to recirculate the cleaners back to the inlet of the heat exchanger. The density of the foam rubber balls is designed to be near that of the cooling water, such that separation of the balls from the water flow in the "water box" at the inlet to the heat exchanger tubes will be minimized. Contamination of the foam rubber balls by silt or other solid materials is known to increase the density and may affect the performance of such cleaners, however.
The requirements for on-line and off-line tube cleaners are quite different. Off-line cleaners can be placed in and forced through individual tubes. Ability to enter a tube and be evenly distributed to all the tubes of a heat exchanger from a flowing stream is not a requirement. Also, the pressure required to push an off-line cleaner through the tube is not limited to the operating pressure drop of the heat exchanger. For example, U.S. Pat. No. 3,939,519 proposed a cleaning plug which includes an elongated core body and a plurality of spaced scraper discs along the body. This cleaning body is not suitable for on-line cleaning, because it would not enter a tube from a flowing stream and would require excessive pressure to force the cleaner through a tube.
During operation of cooling water systems, debris may accumulate in the system. This debris may be a small object such as an aquatic plant or animal or a remnant of the construction material of the system, for example. It is desirable that tube cleaners be able to pass through a tube even when small items of debris are present. This is a consideration in determining the optimum size of a tube cleaner.
For the tube cleaners to be re-circulated through the tubes while on-line, it is necessary that the entry pressure of a tube cleaner into a tube not be excessive. If the entry pressure is excessive, the tube cleaners will accumulate in the manifold or water box upstream of the tubes and this could cause restricted flow through the heat exchanger. After a tube cleaner begins flowing through a heat exchanger tube, it is also important that the pressure required to drive the tube cleaner through not be excessive. Excessive pressure drop would increase the probability that a tube cleaner would become stuck and plug a tube, thereby decreasing the efficiency of the heat exchanger. Some pressure drop is required, however, to ensure that the cleaner is contacting the walls of the tube and applying mechanical force to remove any film material present on the surface of the tube.
On-line tube cleaners are designed to be re-circulated many times through a heat exchanger. Each time they are subject to damage by the pump, to frictional wear, and to flexure. Therefore, high-impact toughness and resistance to abrasion are important properties of a hard-body tube cleaner. Resistance to failure upon repeated flexure is important for any flexible component.
With all types of on-line cleaners, means for removing the cleaners from the stream downstream of the heat exchanger is required. The patent literature for devices to strain or screen the cleaners from a liquid stream is extensive. When the sponge ball cleaners are used, it is necessary that the screen devices have narrow spacing, to assure that the cleaners will not deform and flow through the screen. This narrow spacing significantly increases pressure drop through the screen. There is need for a hard-body cleaner which can be removed from screens having wider spacing.
U.S. Pat. No. 4,569,097 discloses variable density or constant density tube cleaners for on-line use which have substantially neutral buoyancy in cooling water when it enters a heat exchanger. Upon exiting from the heat exchanger tubes, the variable-density tube cleaners return to either positive or negative buoyancy. Skimmer means intercept tube cleaners having positive buoyancy from the upper portion of flow in an open once-through water system. The tube cleaners are re-circulated through the tube bank repeatedly. Constant-density tube cleaners having a flotation member and a cleaning member are disclosed in FIG. 17 of the patent. The flotation member and cleaning member are proportioned to provide for approximately neutral buoyancy in the cooling liquid. The material from which the flotation member is to be formed is not specified. The cleaning member is of an elastomer of high abrasion resistance and high flex failure resistance, such as polyurethane. The cleaning member may be fastened to the flotation member by a molded retaining means integral with the flotation member. The flotation member is a sphere and has a diameter less than the inside diameter of a tube which is to be cleaned. The cleaning member is a disk which has a greater diameter than the inside diameter of the tube, such that the cleaning member provides a wiping motion of its periphery against the inside diameter of a tube as a pressure difference pushes the cleaner through the tube. The cleaning member has deformability such that it forms a "cup" upon entry into a tube and the material and diameter of the cleaning member are selected so that the cup "wobbles" in it travels through a tube.
U.S. Pat. No. 4,696,318 discloses a washing system for removing and cleaning floating tube cleaners of the type disclosed in U.S. Pat. No. 4,569,097 from a body of water.
Tube cleaners having a constant density for on-line cleaning of the tubes of heat exchangers are needed which will be recoverable either by a strainer or by surface collection, can be re-circulated many times through pumps and tubes without failure, will evenly distribute over the tubes of large heat exchangers for uniform cleaning of all tubes, will not become lodged in a tube, will clean effectively, will have a density selected to afford recovery from water by surface collection if desired and will be economical to construct.